Counts locally-addressed datagrams that were received intact but then discarded because their upper-layer protocol field wasn't recognized. Since this only counts traffic addressed to the device itself, a rise here, unlike ifInUnknownProtos which is per-link, points at something specifically targeting the gateway's own IP with a protocol it doesn't handle, worth checking for misconfigured monitoring agents or unusual protocol probes. Example: a spike in ipInUnknownProtos coinciding with a new monitoring tool being pointed at the gateway's IP suggests that tool is using a protocol the gateway's IP stack doesn't support.